Normal Parturition - Anatomy & Physiology

BACK TO REPRODUCTIVE SYSTEM BACK TO PARTURITION

• The Process by which the conceptus (foetus, placenta and placental membranes) are expelled from the Uterus requires:
  • Cervical Softening
  • Coordinated Myometrial Contraction
  • Voluntary contraction of abdominal muscles

Placental Expulsion

In most species, expulsion of foetal membranes quickly follows expulsion of the foetus.

• After the birth of the young, regular abdominal contractions largely cease.
• Myometrial contractions persist.
  • Decreased amplitude, but become more frequent and less regular.
  • Important for dehiscence and expulsion of foetal membranes.
• Waves of contractions from uterus to the cervix persist.
  • Act in a peristaltic fashion in the cow and sow in the reverse direction.

Placental Changes

During the last 5 days of gestation, there are changes in the placenta.

• Collaginisation of the placentome.
• Flattening of maternal crypt epithelium.
• Leucocyte migration and increased activity.
• Reduction of binucleate cells in the trophectoderm.
• Weakening of the acellular protein layer between cotyledonary and caruncular epithelium.

Contractions

• Open endometrial crypts
• Foetal villi have shrunk due to the escape of blood from the foetal side of the placenta when the umbillical cord ruptures.
• Myometrial contractions aid exsanguination of the placenta.
• Separation of foetal membranes.
• Apex of the allantochorionic sac becomes inverted.
• As the sac is 'rolled' down the uterine horns, foetal villi are drawn out of the crypts.
• When a large portion becomes detached and inverted, it forms a mass in the maternal pelvis.
  • Stimulates reflex contractions of abdominal muscles.
  • Completes expulsion of the allantochorionic sac.
• In polytocious species, dehisence and expulsion of foetal membranes are interspersed with births of the young.
  • Only expulsion of the last afterbirth stimulates abdominal contractions.
• The final stage of allantochorionic expulsion lasts 1 hour (mare) - 6 hours (cow).
• Domestic animals normally eat the afterbirth.
  • Not the mare or camelids.

• Expulsion of the foetus requires that the chorionic villi become dislodged from the crypts of the maternal side of the placenta.
  • Brought about by powerful vasoconstriction of arteries in the villi.
    • Vasoconstriction reduces pressure and allows villi to be released from crypts.

Species	Mare	Cow	Ewe	Sow	Bitch
Stage 1: Contractions and Cervical Dilation	1-4 hours	2-6 hours	2-6 hours	2-12 hours	6-12 hours
Stage 2: Foetal Expulsion	12-30 minutes	30-60 minutes	30-120 minutes	150-180 minutes	6 hours (up to 24 hours with large litters)
Stage 3: Placental Expulsion	1 hour	6-12 hours	5-8 hours	1-4 hours	Placenta Exits with Foetus